The field of the invention relates to frequency or period to amplitude conversion for producing a signal having an amplitude corresponding to the output frequency of a sensor.
In many applications, such as automobile applications in particular, sensors provide a frequency signal having a frequency related to a measured parameter. For example, a speed sensor provides a signal having a frequency related to either the vehicular speed or engine speed, a manifold pressure sensor provides a signal having a frequency related to engine manifold pressure and so on. It is necessary to convert the frequency signal into an amplitude signal for signal processing. Examples of signal processing include onboard engine control and also diagnostic testing. The amplitude value may be either a voltage related to frequency or a digital word related to frequency.
In one approach to frequency to voltage conversion, as disclosed in Japanese patent application No. 0106276, one period of a frequency signal to be measured is integrated utilizing analog integrators and timing circuits In another approach as disclosed in U.S. Pat. No. 4,197,508 issued to Takaoka, the period of one frequency signal is applied to a frequency divider and a logic circuit to control the charging and discharging of capacitors. A voltage is thereby obtained proportional to the input signal. One disadvantage of these approaches is the inaccuracy caused by temperature dependent drift of the analog devices.
Still another approach is disclosed in U.S. Pat. No. 3,944,935 issued to Plant wherein generated pulses are counted for a time proportional to input signal frequency. The counter output is applied to a digital to analog converter for generating a dc voltage signal. A feedback signal is then derived from a comparison between the dc voltage signal and a reference signal such that, allegedly, the dc signal is directly proportional to the input signal frequency.
The inventor herein has recognized a disadvantage with all known prior approaches. More specifically, the frequency signal may vary across a frequency range. Prior known approaches are designed to convert any frequency signal from zero frequency to the maximum frequency of the frequency range. Accordingly, the sensitivity to noise of these prior approaches is limited. Further, in the case of digital processing, an excessive number of data bits are required with resulting disadvantages in transferring information and subsequent digital processing.